Distillation is a common method for generating potable water from otherwise unsafe water sources (such as seawater or polluted ground water). With distillation, water is heated to boiling, and the resultant vapor is collected and condensed, producing distilled water.
Known in the art is a method of distillation or desalination of water wherein water is evaporated from an aqueous salt solution when the solution comes into contact with air and distilled water is subsequently retrieved from the thus moistened air by condensation (cf. V. N. Slesarenko “Modern Techniques for Desalination of Sea and Saline Waters” published in 1973 by the Energia Publishers, Moscow, pp., 47-48). However, in this method, a large amount of costly high-calorie heat energy is required to conduct the process (around 600 kcal/kg under atmospheric pressure, which corresponds to approximately 695 watts per 1 kg of desalinated water).
Conventional small distillers typically incorporate an electric heating element to boil water in a tank. A condensing coil mounted above the tank collects the vapor and condenses it. The distilled water is then transferred to a holding tank or cell. These boiler-type distillers, however, require substantial amounts of electrical power to produce relatively little distilled water and are thus highly inefficient. They are also extremely slow, often taking many hours to produce just a few gallons of distilled water. Accordingly, this sort of distiller has not gained widespread acceptance or use.
In addition to boiler-type distillers, thin-film distillers have also been proposed. For example, U.S. Pat. No. 4,402,793 describes a solar-powered, thin film distiller. It contains a plurality of parallel, spaced-apart plates, which are arranged to face the sun. Water to be distilled is supplied to the tops of the plates and guided to flow down the back face of each plate. Sunlight irradiates and heats the plates, causing a portion of the flowing water to evaporate. Vapor condenses along the front side of an adjacent plate, transferring heat to the flow of water on its opposite side and so on. Condensate generated along the front sides of the plates is separately collected at the bottoms of the plates. Although this distiller provides some advantages because it uses solar energy instead electricity, its design is very complicated and efficiency of distillation is small.
French Pat. No 1,162,054 discloses a distillation process in which liquid to be distilled is passed into a zone where energy is added thereto and then discharged from that zone in the form of a thin film, which is then contacted with a current of distilling vapor that is passed across the surface of the film. The big disadvantage of these patents is significant expenses of the energy.
Many types of evaporators have been used to produce fresh water by distillation. But in the past, the prior art types of evaporators have proved costly and troublesome when used continuously on a large scale. At temperatures over 160° F., seawater and many brackish waters deposit scale (incrustation of insoluble chemical compounds, especially calcium and magnesium), which interfere with the operation of the evaporator. Hot seawater is exceedingly corrosive. Most prior art methods of obtaining fresh water from seawater have been most inefficient. Modern desalination processes try to avoid the inefficient operations of the prior art by using of low temperature desalination for removing fresh water from seawater.
In another known process for recovering water from the atmosphere (see U.S. Pat. No. 4,197,713 and No. 4,219,341) the water vapor contained in the atmosphere is condensed on plastic sheets cooled by nightly radiation and the drops of water formed are collected. The yield of this process is very low.
An innovative heat tower process referred to as “Dewvaporation” has been investigated and is now operational at Arizona State University. The Dewvaporation technique uses a carrier-gas to evaporate water from saline feeds and dew-form pure condensate at a constant atmospheric pressure. The heat released by dewfall condensation on opposite sides of heat transfer walls supplies the heat needed for evaporation. Since only a small amount of external heat is needed to establish temperature differences across the wall and since the temperature of the external heat is versatile, the external heat source can be from waste heat, from solar collectors or from fuel combustion. The unit is constructed out of thin water-wettable plastics and operated at pressure drops about 0.1 inches of water (see Beckman, J. R., Final Report, Innovative Atmospheric Pressure Desalination, No 52, US Department of Interior, Bureau of Reclamation, 1999). But productivity of this atmospheric pressure desalination is small because processes of humidification and dehumidification are not efficient.
Another method of the atmospheric pressure distillation or desalination of water has been proposed by Maisotsenko in U.S. Pat. No. 4,350,570. According to this method of distillation evaporation of water from the aqueous salt solution through contact with air is conducted by using a primary and a secondary airflow. The primary airflow is supplied to a cooling zone of a vessel, while the secondary airflow and the aqueous salt solution are delivered to an evaporation zone, wherein the secondary airflow is moistened by the water evaporating from the aqueous salt solution by virtue of the temperature difference. During the course of absorbing the moisture, the secondary airflow acts to cool the primary airflow passing through the cooling zone. The secondary airflow is obtained by withdrawing between 20 and 90 volume percent from the primary airflow, after it has passed through the cooling zone of a vessel. Condensation of the water vapor is effected by conveying the secondary air flow to a condenser. This system is still not as efficient as desired, and requires a divided vessel and condenser.
Evaporative cooling may be used to cool air or any fluid below its wet bulb temperature and up to its dew point temperature. This Maisotsenko Cycle is taught in various patents, including U.S. Pat. Nos. 5,453,223; 6,497,107; 6,705,096; 6,776,001; and 6,779,351 (all incorporated herein by reference).
British Pat. No 549,519 discloses a high vacuum distillation apparatus, which has highly polished vaporizing and condensing surfaces that are separated by a substantially unobstructed space.
Inventor William Zebuhr has developed the thin film vacuum distillation system (see U.S. Pat. Nos. 6,423,187 and 6,689,251) and rotary evaporator and condenser for use in a vapor compression distiller (see U.S. Pat. No. 6,261,419 and No. 6,592,338). The different applications, which are used the water vapor compression cycle for producing the chilled water, were protected by next U.S. Pat. No. 2,096,147 “Refrigeration”, U.S. Pat. No. 2,129,098 “Steam Jet Refrigeration Apparatus”, U.S. Pat. No. 3,563,049 “Aspirator and Circulating Cooling Apparatus”, U.S. Pat. No. 3,695,208 “Food Storage Apparatus for Use in Water-Borne Vessels”, U.S. Pat. No. 4,102,392 “Low Energy Consumption Air Conditioning System”, U.S. Pat. No. 4,576,014 “Produce Vacuum Cooler with Improved Venting”, U.S. Pat. No. 4,607,491 “Cooling Trap for Vacuum”, U.S. Pat. No. 4,723,415 “Direct Water Evaporating Cooling System”, U.S. Pat. No. 6,329,005 “Rapid Cooling of Sealed Package”, U.S. Pat. No. 6,427,453 “Vapor-Compression Evaporative Air Conditioning Systems and Components”, U.S. Pat. No. 6,484,527 “Method for Operation a Refrigerating System”.
There are today some companies, which successfully exist on the market, which utilize the water vapor compression cycle for producing the distilled water (for example, Ovation Products Corporation, USA), chilled water for cooling plant (for example, LEGO Company, Denmark), binary ice (for example, Integral Energietechnik GmbH, Germany and I.D.E. Technologies Ltd., Israel) and etc.
The underlying principle of vapor compression distillers is that, when the pressure of a vapor is increased, its saturation temperature rises. In conventional vapor compression distillers, vapor produced in an evaporator (vacuum chamber) is removed, compressed and returned to the condenser, where it condenses, producing a distillate. A compressor is used to reduce the pressure within an evaporator to a sub-atmospheric level causing the evaporation of vapor from a solution, which acts to take the heat of vaporization from the solution, thereby reducing the water temperature. This chilled water can be used for different air conditioning and cooling systems. Vacuum-process technology producing chilled water needs no refrigerant of the conventional kind, but water from the process itself is used to generate cooling. Furthermore, the heat of vaporization that is emitted as the vapor condenses may be used to heat and thus evaporate the liquid being distilled.
But all existing vacuum-process technologies of the water vapor compression cycle for producing distilled water have essential disadvantages. First of all it is not efficient when the heat of condensation of water vapor is used as heat for evaporation. Usually the existing systems use air and water-cooling condensers (with or without cooling towers) and sometimes evaporative cooling condensers. Here the air or water being cooled cannot be cooled lower than the wet bulb temperature of outside air. Therefore pressures of condensation are high and this increases consumption of energy by the compressor and reduces the productivity of the water vapor compression cycle. In addition, all or part of the heat of condensation is lost to the atmosphere without recovery. The heat transfer rate in the condenser and evaporator is low.
Accordingly, it is desired to improve water distillation methods and apparatus.